Refractory mixture for cast metal molds



Patented Mar. 4, 1941..

2,233,700 REFRACTORY MIXTURE FOR. CAST METAL MOLDS Cornell Joel Grossman, Millburn, N. J.

No Drawing. Application July 8, 1940, Serial No. 344,435

4 Claims.

This invention relates to refractory materials for use in the manufacture of molds for the casting therein of cast metals, and more particularly for the forming therein of cast metallic dentures. The mold into which the molten metal is cast must be relatively strong and highly refractory, non-reactive towards the molten metal, and must be smooth surfaced and substantially free from volume changes within the temperature range from atmospheric temperatures to about 1500" F.

One of the objects of the present invention is to provide a refractory composition meeting these exacting requirements of mold material. Another object is to provide an improved refractory l5 material for use in the forming of denture molds in the manufacture of cast metallic dentures. Still another object is to provide a refractory mixture adapted to be plasticized and molded into a casting mold for metallic dentures. Other objects and advantages will be apparent as the invention is more fully hereinafter disclosed,

In the manufacture of molds for east metallic dentures, the refractory composition is first formed into a plastic mass capable of setting within a relatively short time interval into a relatively strong mass and is applied, preferably in a plurality of relatively thin coatings onto the surface of a wax or other pattern material shaped to the configuration desired, building up thereon a satisfactory depth of the refractory. The mold thereby obtained is backed up by other refractory material to strengthen the same and is thereafter slowly heated to the casting temper-' ature and the molten metal then is centrifugally cast therein.

In such procedure, it is highly essential that the refractory material surfacing the pattern material shall deposit thereon in a smooth surface condition that will define accurately the exterior 40 surface of the pattern. It is also essential that the refractory material comprising the mold shall evidence substantially no change in volume on being heated to the metal casting temperature. Further, it is essential that the mold material 5 in its as cast condition on the pattern and in its heated condition for receiving the molten metal shall be relatively strong to prevent injury to the mold and also shall be inert'or non-reactive with respect to the molten metal cast therein.

60 I have discovered that a refractory mixture consisting of silica and compounds known in the art as double Mg-silicates of Zr and Ti, when mixed together in the proportions 95 to 70% S102 and 5 to 30% double silicate; may be heated throughout the range of temperatures atmos- 'pheric to maximum metal casting temperature (approximating 1500 F.) without substantial change in volume.

I have further discovered that by limiting the particle size of the double silicate to approxi- 5 mately that known in the art as fiour? size.(below about 200 mesh) and by limiting the silica to sizes approximating -80% passing 20 mesh and 20-40% passing 200 mesh, the resulting mixture when bonded together as hereinafter de- 1 scribed will present a substantially smooth, even surface defining accurately the surface configuration of the pattern on which it has been cast. The chemical inertness of this refractory mixture renders it useful in the casting of any of the 15 metallic compositions commonly employed in the forming of metallic dentures.

The refractory mixture hereinabove described may be bonded by a plurality of bonding agents, as one skilled in the art may perceive; Prefer- 20 ably, a bonding agent substantially free from carbon and sulfur or thermally decomposable sulfur compounds is employed to avoid contamination of the cast metal in the mold. One of the most satisfactory of bonding agents is phosphoric acid or a mixture of phosphoric acid and aluminum phosphate. Phosphoric acid alone appears to react at least superficially with the silica and double silicate compounds forming the refractory mixture to form complex phosphate compounds 30 therewith imparting setting properties to the mixture when in the plastic state, the:- setting time being variable from a few: seconds 'to many minutes, depending primarily upon the amount of phosphoric acid present, the amount of water 35 employed therewith and uponthe amount of double silicate present.

Additions of aluminum phosphate and other water or phosphoric acid-soluble metal phosphates, appear generally to increase the strength 4.0 of the refractory mixture in its as cast and preflred condition and appear generally to beneficiate the obtainance of the desired smooth surface characteristics of the mold and the surface hardness of the mold in the fired condition. -The 45 aluminum phosphate may be employed in addition to the phosphoric acid or in replacement of equivalent amounts of the phosphoric acid, as may be desired, without substantial effect except as to setting time and offers a convenient means 60 for controlling the setting time of the refractory mixture,

As one specific embodiment of the present invention, but not as a limitation thereof, the double silicate compound to be employed, for exampie, zirconium-magnesium silicate is first prefired to maximum density at temperatures approximating the maximum metal casting temperature employed (approximately 1500 F.) and is thereafter ground to approximately flour size (approximately 200 mesh and below) The silica (S102) also is similarly fired to maximum density at the same maximum metal casting temperature and then is ground and sized to pass about a 20 mesh screen with from 20-40% of the same passing about 200 mesh.

The two pre-fired materials then are formed into a refractory mixture preferably using 1 part (by weight) of the double silicate to 4 parts (by weight) of the silica, althoughgood results may be obtained by employing a mixture containing double silicate and silica within the percentages ranges 5-30% double silicate and 95-70% silica.

In the manufacture of cast denture molds, relatively small amounts of refractory are employed, even with the largest dentures. This small amount of refractory material, approximating pound of material, is then formed into a plastic mass using a phosphoric acid solution of approximately 8% phosphoric acid. The strength of the phosphoric acid solution employed and the volume thereof employed in the forming of this plastic mass may be widely varied without departure from the present invention. In general, with any given strength phosphoric acid solution, the setting time is shorter the stifier the plastic mass and with any given plasticity the setting time is shorter the higher the phosphoric acid concentration. With any given denture size there is a wide variation in setting time for optimum results, as one skilled in the art will recognize.

The plastic mass then flowed into the mold, preferably in a plurality of coatings or layers to the desired depth or thickness and preferably also with vibration to insure the elimination of all surface voids and cracks and to obtain the maximum surface density at the pattern face.

After the thus deposited refractory has set to its maximum hardness the mold thus obtained is then carefully dried at relatively low temperatures and when completely dry is slowly heated to the casting temperature of the metal forming the denture and maintained at this temperature for a sumo-lent time interval .to insure thorough degasification and consolidation of the refrac tory mixture. 7

In general, in the manufacture of cast metallic dentures the molten metal is centrifugally cast within the mold. With any given metal composition the mold preferably is pre-heated to such a temperature as will maintain the molten metal in a fiui-d condition wtihin the mold for a time interval sufficient to permit the metal to flow into all of the intricate folds, convolutions, cracks and fissures of the mold surf-ace.

It is preferable, to avoid the development of expansion and contraction cracks in the mold in cooling the pre-fired mold to the temperature found suitable as a pro-heat temperature in the metal phosphate in the refractory mixture replacing part to nearly all of the phosphoric acid employed above improved results with respect to surface smoothness in the mold, strength of mold in the as oast-and as fired condition will be obtained'which for exceedingly intricate patterns is highly desirable.

Such additions preferably, are made by dissolving freshly precipitated metal hydroxide, such as aluminumhydroxide, in phosphoric acid,, maintainlng the acidity of the solution at least sufficient to retain the metal phosphate in clearsolution. This solution may be used in replacement of the phosphoric acid solution hereinabove disclosed or may be admixed therewith in various proportions from a small amount up to V. V

phates or phosphate compounds formed by the reaction of the phosphoric acid present in the silica and-double silicate refractory mixture employed, thereby to provide secondary cementing bonds interiorly within the molded structure on firing.

Having hereinabove described the present invention generically and specifically and given a specific embodiment of the same, it is believed apparent that the same may be widely varied without essential departure therefrom and all such modifications, adaptations and departures therefrom are contemplated as may fall within the scope of the following claims:

What I claim is:

1. The method of forming a refractory mold for-cast metallic dentures which comprises forming a mixture comprising at least 60% silica and 5-30% of a double Mg silicate compound of one of the metals zirconium and titanium, the said silica having a particle size passing about 20 mesh with from 20-40% thereof passing 200 mesh andthe said silicate having a particle size passing about 200 mesh, forming the mixture into a plastic mass with phosphoric acid or sim ilar binder, flowing the plastic mass over the surface of a pattern in a plurality of coatings to the depth and thickness desired, allowing the coatings toset, drying the coatings and then .slowly heating the same to the desired pre heat temperature of the mold before casting the molten metal therein;

2. A refractory mixture for use in the forming of molds for cast metal, said mixture comprising from 95-70% silica having a particle size passingabout 20 mesh with 20-40% of the same passing about 200 mesh and from 5 to 30% zirconium magnesium. silicate having a particle size passing about 200 mesh.

3. A refractory mixture for use in the forming of molds for cast metal, said mixture comprising from 95-70% silica having a particle size passing about 20 mesh with 20-40% of the same passing about 200 mesh and from 5-30% titanium magnesi-um silicate having a particle size passing about 200 mesh.

4. A refractory mold composition bonded by a refractory metal phosphate of the group aluminum and magnesium and phosphoric acid, comprising 5 to 30% zirconium magnesium silicate and at least 60% of silica.

CORNELL JOEL GROSSMAN. 

